1. Field of the Invention
This invention relates to a method for increasing the efficiency of a board plant. More particularly, the present invention relates to utilizing the water vapor and carbon dioxide present in the flue gas of a board plant to produce syngas.
2. Description of the Background Art
In the production of gypsum board, direct fired natural gas burners are used to deliver heat directly to wet gypsum boards as they continuously pass through a dryer. Typically gypsum boards are 25-33% moisture at the “wet” end of the dryer and less than 1% moisture at the dry end. Almost all of the water contained in the wet board is evaporated out of the board through the heat delivered from the combustion of the natural gas in a natural gas burner. There is direct contact between this gas and humidified recirculated gas which is drawn over the wet gypsum boards causing them to heat up and allowing the evaporation of the water. As a result of the evaporation, the water content of the air within the dryer is often 0.3 to 0.4 pounds of water for every pound of air. This water is passed up the stack of the board plant and out into the environment.
The evaporation of water from the wet boards is energy intensive. Most of this energy is supplied from natural gas. The natural gas is burned to produce thermal energy required for drying the gypsum board. Modern gypsum board plants require around 200,000,000 Btu/hr. or approximately 200,000 cubic ft. of natural gas per hour. This is a tremendous amount of natural gas.
The large amount of natural gas required is problematic due to the instability in the natural gas market. High demand in 2005 to 2006 caused gas pricing to nearly quadruple in a few years. In recent years, gas pricing has peaked at about $13.50/million BTU. By contrast, energy prices for coal/pet coke and biomass were generally less than $2.40 to $4.8/million BTU. Biomass has an additional advantage in that many conservation minded communities are now charging tipping fees to encourage the beneficial use of yard wastes and tree trimmings. Other energy sources, such as coal, wood and waste fuels have also become more economically viable in recent years.
Thus, there exists a need in the art for ways of reducing the amount of natural gas used in board dryers. And there likewise exists a need in the art for board drying methods that utilize less expensive energy sources, such as biomasses, coal, wood or waste fuels. Gasification is a known process whereby feed materials such as biomass, coal, wood, or waste fuels are used to produce synthesis gas, or syngas. The background art illustrates several different gasification techniques.
For example, U.S. Pat. No. 6,877,322 to Fan discloses the recycling of CO2 to be used in gasification processes. The CO2 is in the form of compressed exhaust gas generated from a gas turbine compressor system. 02 and steam are also constituents of the exhaust gas which is fed into the gasifier. The exhaust gas is fed to the gasifier via a conductor (for example, a hose or tubing). As the constituents react in the gasifier, syngas is produced. The syngas comprises carbon monoxide (CO) and hydrogen (H2).
Furthermore, U.S. Pat. No. 5,724,805 to Golomb discloses the recovery of CO2 from a gas turbine arranged to combust a fuel. Substantially pure oxygen gas and CO2 are fed into a gas turbine to produce an exhaust gas comprising water and carbon dioxide. The gas turbine exhaust is used to generate steam in the heat recovery steam generator (“HRSG”) for power generation in a steam turbine. A portion of the steam generated from the HRSG is used as steam input for coal gasification. The flue gas from the HRSG is cooled and water vapor is removed; what is left is pure CO2 which is split into two streams. One stream is recycled and the other is removed from the system as liquid CO2. Ambient air is separated into its components, wherein 02 is partially (one stream) diverted into a coal gasifier, where coal, 02 and steam are converted into synthesis gas. The other 02 stream is sent to a gas turbine combustor, wherein the combustion products are CO2 and H20.
U.S. Pat. No. 5,517,815 to Fujioka discloses a coal gasification generator wherein a high temperature combustion gas is produced from a coal gasifying furnace and supplied to a gas turbine. The gas turbine drives a power generating unit, wherein the exhaust gas produced is fed to a boiler. The water vapor produced from the boiler is heated by a heat exchanger and is subsequently supplied to a gasifying furnace as a gas.
Additionally, U.S. Pat. No. 4,963,513 to Marten discloses a process by which coal is reacted in a gasification zone in the presence of an oxygen and sulfur containing atmosphere. From this process, carbonaceous char and a crude coal gas stream is produced. The carbonaceous char and gypsum may be combined to form a feed mixture such that they are reacted in a reaction zone under reducing conditions to produce a sulfur-dioxide-containing gas stream which contains weaker SO2, which is removed from the reaction zone. This sulfur-dioxide gas stream can be recycled back to the coal gasification zone where it can be mixed with air to provide the oxygen-equivalent required for the coal gasification.
Finally, another well known gasification technique was originally developed by Albert L. Galusha and is now known as Wellmen-Galusha gasification. This process is generally described in U.S. Pat. No. 2,440,940 to Galusha.
Although each of the above referenced inventions achieves its respective objective, none of the background art involves utilizing gasification techniques in conjunction with a board dryer. More specifically, none of the background art discloses utilizing the flue gas from a gypsum board plant as a constituent of gasification.